1. Field of the Invention
The present invention concerns methods for magnetic resonance imaging that can be used in medical engineering to examine patients. The present invention in particular concerns methods to correct distortions or deformations in the phase coding direction that can occur given a use of echoplanar sequences (known as EPI sequences) and that can negatively affect image quality.
The present invention likewise concerns a magnetic resonance system for implementing such methods.
2. Description of the Prior Art
Magnetic resonance tomography (MRT), which is used for magnetic resonance imaging, is based on the physical principle of nuclear magnetic resonance. In magnetic resonance tomography, an examination subject (a patient, for example) is exposed to a constant, strong magnetic field. The nuclear spins of the atoms in the subject, which were previously oriented at random, thereby align. Radio-frequency waves can excite these aligned nuclear spins into a precession movement that causes the actual measurement signal in the magnetic resonance tomography apparatus. The measurement signal can be acquired with suitable reception coils. The examination subject can be spatially coded in all three spatial directions by the use of non-homogeneous magnetic fields that can be generated by gradient coils.
In one possible method to generate magnetic resonance images (MR images), a slice is initially selectively excited in the z-direction, for example. The coding of the spatial information in the slice takes place via a combined phase and frequency coding by means of two orthogonal gradient fields that (in the example of a slice excited in the z-direction) are generated by gradient coils in the x-direction and y-direction. The imaging sequence is repeated for varying values of the phase coding gradient, wherein the nuclear magnetic resonance signal is acquired multiple times in each sequence pass in the presence of the readout gradient. A number matrix in a mathematical domain known as raw data space or k-space is obtained in this way. A magnetic resonance image of the excited slice can be reconstructed from this number matrix through a Fourier transformation.
An additional method to generate magnetic resonance images is known as echoplanar imaging (EPI). Multiple phase-coded echoes are used to fill the raw data matrix. After a single (selective) radio-frequency excitation, a series of echoes is generated in the readout gradient that are associated in raw data space with different lines in the excited slice by a suitable modulation of the phase coding gradient.
An example of an echoplanar pulse sequence is shown in FIG. 1. After an excitation pulse and a refocusing pulse, multiple gradient echoes are generated by a sinusoidally oscillating frequency coding gradient GR in the readout direction and phase coding. In this representation the phase coding takes place by small gradient pulses (known as blips) of the phase coding gradient Gp in the region of the zero crossings of the oscillating frequency coding gradient GR, and this leads to a wandering traversal of the raw data space (as shown in FIG. 2). EPI alternatively can be implemented as a Cartesian EPI with a rectangular curve of the readout gradient GR, for example, or as a non-Cartesian EPI (for example spiral EPI or radial EPI).
EPI sequences have extremely short measurement times, typically of 30-50 ms for one MR image acquisition per 2D slice. Such sequences are particularly suitable in functional imaging and in perfusion and diffusion measurements since movement artifacts (due to breathing or pulsing motion of blood or fluid, for example) can be drastically reduced. A problem with such fast imaging methods, however, is their high sensitivity to B0 field distortions or induced susceptibilities, since the readout time per excitation is significantly longer compared to other methods that acquire only portions of a line or one line in raw data space.
Methods are known with which information about the inhomogeneities of the magnetic field can be derived from two k-space trajectories that are adjacent to one another, but displaced in the phase coding direction. A field map or a displacement map can be determined from this information. Such a field map shows magnetic field distortions or magnetic field shifts, while a displacement map includes the original positions (or the offset relative to the original position) of the image points of the magnetic resonance images that were distorted or displaced in the phase coding direction due to these magnetic field shifts or magnetic field inhomogeneities. Such a method to create displacement maps is what is known as the PLACE method (Phase Labeling for Additional Coordinate Encoding). This PLACE method is described in detail in the publication by Qing-San Xiang and Frank Q. Ye with the title “Correction for Geometric Distortion and N/2 Ghosting in EPI by Phase Labeling for Additional Coordinate Encoding (PLACE)”, Magnetic Resonance in Medicine 57:731-741 (2007), and is therefore only briefly outlined in the following using FIGS. 3 and 4. In the PLACE method, two echoplanar imagings with Cartesian scanning [sampling] are implemented in succession. The first scan (shown in FIG. 3) is implemented in a typical manner in that a k-space trajectory is traversed in a wandering pattern. For clarity, the number of lines in the phase coding direction amounts to only nine in FIG. 3 and is significantly higher (for example 32-256) in real echoplanar imagings. After this, a second echoplanar imaging is implemented which is shifted by one or more lines in the phase coding direction, for example. FIG. 4 shows such a k-space trajectory which was shifted by two lines, as represented by the double arrow 41. Physically, the gradient range that is thus added generates a relative phase ramp across the examination subject and directly codes the undeformed original coordinates in the phase coding direction of each image point in a phase difference between the two distorted complex images which were acquired from the first imaging according to FIG. 3 and the second imaging according to FIG. 4. The phase information is then used in order to map the distorted signals to their original locations. Expressed in a different way, the image point distortion (image point deformation or image point shift) can be determined from the phase information of the offset k-space trajectories of two pre-interventional EPI images. From this a displacement map can be determined which can be applied to the acquired images.
Typically, 200-300 images are acquired per volume in a functional EPI measurement (typically 40-60 slices per volume). DE 10 2008 007 048 B4 describes a method for dynamic distortion correction in EPI measurements in which immediately successive image acquisitions differ in an alternating or otherwise periodic manner with regard to phase information, phase coding direction or with regard to the echo time, and due to this difference a field map and/or a displacement map with which at least one distorted result image is corrected is respectively calculated from pairs of immediately successive image acquisitions. According to one embodiment, corresponding field maps or displacement maps are determined with the use of the previously described PLACE method from pairs of immediately successive image acquisitions.
As previously described, the two EPI measurements that are used in the PLACE method differ in that k-space is scanned in two trajectories that are shifted in the phase coding direction, and therefore cannot be applied for non-Cartesian scans (for example spiral-shaped or radial scans as are known from Gary H. Glover and Christine S. Law in “Spiral-In/Out BOLD fMRI for Increased SNR and Reduced Susceptibility Artifacts” in Magnetic Resonance in Medicine, 46:515-522 (2001), for example). Furthermore, in the PLACE method corresponding lines in k-space of the two EPI measurements are acquired at different echo times. Different amplitudes (and thus different magnitude images) thereby result. The magnitude images therefore are not directly comparable with one another.